1. Field of the Invention
The present invention relates to a technique for forming a thin film and a technique for manufacturing an element utilizing plasma.
2. Description of the Related Art
The market of liquid crystal displays using thin film transistors (hereinafter, also referred to as “TFT”) has been expanded. TFTs are one kind of field effect transistors and include a thin film of a semiconductor layer for forming a channel formation region. The semiconductor layer is formed of amorphous silicon or polycrystalline silicon.
Amorphous silicon TFTs have a problem that time for writing to a pixel is insufficient with increase in a panel size and density of the pixel. It has been considered that polycrystalline silicon TFTs solve the above-mentioned problem because the polycrystalline silicon TFTs have higher electron field-effect mobility than the amorphous silicon TFTs (for example, refer to Patent Document 1: Japanese Published Patent Application No. 2000-150888).
In addition to the above two kinds of semiconductor layers, TFTs in which a channel formation region is formed using a microcrystalline silicon semiconductor are known (for example, Patent Document 2: U.S. Pat. No. 5,591,987 and Non-Patent Reference 1: Toshiaki Arai, et al., SID 07 DIGEST, 2007, pp. 1370-1373). Microcrystalline silicon is formed by a plasma CVD (Chemical Vapor Deposition) method, similarly to amorphous silicon. For example, as a method for manufacturing a microcrystalline silicon layer by a plasma CVD method, there is a method in which a high frequency in the VHF (very high frequency) band of 30 MHz or higher is utilized (refer to Patent Document 3: Japanese Patent No. 3201492).
Incidentally, a liquid crystal display is manufactured by processing a plurality of panels over a large-sized glass substrate called mother glass and cutting the substrate into the panels having sizes corresponding to screens of television sets or personal computers lastly. This is because cost per one panel is reduced by taking out a plurality of panels from one mother glass. In the market of liquid crystal displays, screen sizes (panel sizes) have been increased rapidly, and selling prices have been reduced rapidly. In order to improve productivity along with increase in sizes and reduction in cost, increase in the size of mother glass has been promoted.
The size of the typical glass substrate in around 1991, so-called first generation, was 300 mm×400 mm. After that, the mother glass has been enlarged to have sizes of 400 mm×500 mm (second generation), 550 mm×650 mm (third generation), 730 mm×920 mm (fourth generation), 1000 mm×1200 mm (fifth generation), 2450 mm×1850 mm (sixth generation), 1870 mm×2200 mm (seventh generation), 2000 mm×2400 mm (eighth generation), 2450 mm×3050 mm (ninth generation), and 2850 mm×3050 mm (tenth generation).